1. Field of Invention
The present invention relates, generally, to automotive powertrain systems and, more specifically, to a hydraulic mode-stable powertrain transfer case.
2. Description of the Related Art
Conventional automotive vehicles known in the art include a powertrain system in rotational communication with one or more drivelines used to drive the vehicle in operation. Typically, the vehicle includes a pair of drivelines, each defined by a respective pair of opposing wheels. The powertrain system includes a propulsion system adapted to generate and selectively translate rotational torque to one or more of the wheels so as to drive the vehicle. To that end, in conventional automotive powertrain systems, the propulsion system is typically realized as an internal combustion engine in rotational communication with a transmission. The engine generates rotational torque which is selectively translated to the transmission which, in turn, translates rotational torque to one or more of the drivelines. The transmission multiplies the rotational speed and torque generated by the engine through a series of predetermined gear sets, whereby changing between gear sets enables the vehicle to travel at different vehicle speeds for a given engine speed.
In so-called “four-wheel-drive” or “all-wheel-drive” powertrain systems, both drivelines may be used to drive the vehicle. To that end, all wheel drive powertrain systems typically include a transfer case disposed in rotational communication with the transmission and adapted to split rotational torque between the drivelines. The transfer case may be spaced from the transmission, or may be integrated with the transmission. Where the transfer case is spaced from the transmission, a driveshaft is used to translate rotational torque from the transmission to the transfer case. Driveshafts are also typically used to connect the transfer case to each respective driveline. Conventional drivelines are typically realized by a differential assembly adapted to receive rotational torque from the transfer case and subsequently split rotational torque between opposing wheels. To that end, each driveline also typically includes a pair of continuously-variable joints disposed in torque translating relationship with the differential and each respective opposing wheel.
Depending on the specific configuration of the powertrain system, the percentage of torque split between the drivelines may vary. Moreover, depending on the vehicle application, the transfer case and/or driveline(s) may be configured to selectively interrupt rotational torque to one of the drivelines under certain operating conditions. Specifically, the powertrain system may be configured such that the vehicle can be selectively operated in “two-wheel-drive” or in “four-wheel-drive”. Moreover, the powertrain system may be configured to automatically and continuously control how much rotational torque is sent to each driveline. Thus, the powertrain system may be configured to send a higher percentage of available torque to one of the drivelines under certain vehicle operating conditions, and a lower percentage of available torque to the same driveline under different vehicle operating conditions. By way of non-limiting example, the powertrain system may be configured such that 80% of available torque is sent to a front driveline and 20% of available torque is sent to a rear driveline until there is a loss of traction or wheel spin, whereby the powertrain subsequently adjusts torque split such that 50% of available torque is sent to each driveline.
Depending on the vehicle application, rotational torque may only be required at both drivelines relatively infrequently. Thus, the vehicle may be designed to operate primarily in “two-wheel-drive” so as to minimize parasitic loss and optimize powertrain system efficiency, or may be selectable between “two-wheel-drive” and “four-wheel-drive” by the vehicle operator, as noted above. Irrespective of the type of vehicle powertrain system employed, it is desirable for the powertrain be configured as “mode stable,” whereby the transfer case is configured to maintain or otherwise remain in a selected operating mode upon vehicle shut-down, which may occur intentionally (such as when the vehicle is parked and the operator keys-off or otherwise exits the vehicle) or unintentionally (such as caused by an electrical system failure or inadvertent keying-off of the ignition during vehicle operation). As such, it will be appreciated that powertrain system mode stability is desirable both when the vehicle is parked and when the vehicle is moving. By way of non-limiting example, if the vehicle were parked on a steep incline and left in four-wheel-drive operating mode, ab inadvertent switching to two-wheel-drive could cause the vehicle to subsequently roll down the incline. This scenario is particularly problematic where the transfer case utilizes one or more hydraulically-actuated clutch assemblies to effect changing between operating modes, as hydraulic clutch assemblies inherently relinquish holding force over time due to bleed down (seal leakage).
Each of the components and systems of the types described above must cooperate to effectively facilitate selective translation of rotational torque to the driven wheels of the vehicle. In addition, each of the components and systems must be designed not only to facilitate improved performance and efficiency, but also so as to reduce the cost and complexity of manufacturing vehicles. While powertrain systems known in the related art have generally performed well for their intended use, there remains a need in the art for a hydraulic mode-stable powertrain transfer case that has superior operational characteristics, and, at the same time, that reduces the cost and complexity of manufacturing vehicles that operate with high efficiency under a number of different driving conditions.